The wood used in paper making consists essentially of cellulose and hemicellulose fibers bound together by a polymeric material called lignin. A goal of the chemically-based wood pulping processes is to separate the cellulose and hemicellulose from the lignin by degrading the lignin into lower molecular weight species that are soluble in water. In a typical pulping process, wood is added to an aqueous solution of an alkali such as sodium carbonate, sodium bicarbonate or sodium hydroxide, and the resulting mixture is heated under pressure to a temperature in the range of 140.degree.-180.degree. C. While the wood lignin may be broken down by reaction with the alkali alone, the process is slow and also results in excessive degradation of the cellulose and hemicellulose which are the desired products. In order to shorten the processing time or lower the processing temperature or both, accelerators such as sodium sulfite or sodium sulfide are usually added to the alkali solution to aid in processing.
The sulfur based accelerators reduce the processing time and produce a product superior to that produced by alkali alone, but these additives also introduce additional problems. For example, for the sulfide (kraft) process, the resulting pulp material is dark and more difficult to bleach, malodorous air emissions may occur, and expensive chemical recovery equipment is needed in order to reduce chemical costs. While the sulfur containing systems do present several difficulties to the paper making industry, they are nonetheless the standard process used because they offer the overall greatest flexibility, low chemical costs and strongest paper. For example, the lignin containing black liquor from the pulping process is burned to produce heat, and the sulfur and caustic are recovered from the burner and recycled. Sulfur recovery, however, is a difficult process and represents a major problem to the industry.
An alternative to the use of sulfur accelerators was described by Holton in U.S. Pat. No. 4,012,280 in which he describes the addition and use of quinone compounds such as napthoquinone, anthraquinone, phenanthrenequinone, anthone and their ring substituted derivatives (collectively called AQ) as accelerators in the wood pulping process. The AQ compounds may be used to replace some, most or all of the sulfur compounds, thereby decreasing or eliminating the odor problems and costs of the sulfur compounds. However, while the AQ compounds solve two problems, they introduce the problem of their own: cost.
The preferred quinone in the Holton invention is anthraquinone. This chemical is retained in the black liquor when it is separated from the pulp product and cannot be economically removed from the liquor. When the black liquor is concentrated and burned for its fuel value, the anthraquinone is destroyed. The Holton invention thus has a non-returnable chemical cost associated with it. Even at the level of 0.1 wt. % of the wood pulp as described in the Holton patent, the cost of the AQ compounds is an economic deterrent to widespread use.
It is an object of this invention to provide a process whereby lignocellulosics, lignin and lignin derived compounds, such as may be found in black liquor obtained by pulping wood, can be reacted to form a mixture of quinone compounds containing benzoquinones, napthoquinones, anthraquinones and phenanthrenequinones which can be added to the pulping process to accelerate the lignin degradation process.